Oh, no you don'tG 10

The first opportunity to get this achievement in the campaign is at the beginning of Act II (Chapter 5). You will find a Ram in the church courtyard. Rams are very tough from the front and do massive amounts of damage if they hit you. However you will do a large amount of damage to them if you hit them in the back. Don't try and tackle a Ram when they are charging at you.

Sam B will need to reach at least level 7 to be able to put 2 ranks in the tackle skill. As ACEs 8s 24 points out in their solution, the higher your level the more health the Ram enemy has.

If you have already got as far as Chapter 5 with Sam B, then you can restart Chapter 5 with a level 8 Sam B. This way you will get a lower health Ram to take down without having to perform a repeat speed run of Act I.
1. Start a new game and select Sam B and start from Chapter 3 with a default level 1 Sam B and you will automatically be leveled up to level 8.
2. Quit.
3. Start a new game and select Sam B and start from Chapter 5, only this time select the Sam B from your Chapter 3 save. You will be warned that this Sam B will be a low level.

You can make it even easier with a friend over System Link or XBL.
1. Get them to start a new game from Chapter 1 with a default level 1 character.
2. Get them to quit
3. Get them to start a new game from Chapter 5 with the level 1 character from their Chapter 1 save.
4. Join their game and make sure you have the setting to scale enemies to your level turned off.
That way you will only be up against a level 1 Ram.